X PREFACE 



quate in this respect : owing to the comparatively limited 

 number of facts then at his disposal, it could not but be 

 what we may call an ideal theor\- ; that is to say, it is 

 founded upon certain principles without inquiring how far 

 they are based upon actual facts. In themselves, such 

 theories can hardly be looked upon as suggestive, for if 

 once the assumed principle is accepted, all the phenomena are 

 thereby explained, and the matter is open to no further 

 doubt. 



Let us assume that the germ contains millions of the 

 primary constituents ('Anlagen') of all the most minute 

 portions of the body ; moreover, that these constituents are 

 always present at the right place and in the right combina- 

 tion during the process of development ; and, further, that 

 they are capable of giving rise in their turn to the parts or 

 organs to which they severally correspond. Such a theory 

 explains everything, or nothing — the premises alone can be 

 attacked. No new problems can arise from it till it has 

 been placed upon a sound basis ; the premises must be 

 shown to be correct, and it must be proved that the germ is 

 actually composed of primary constituents, which by some 

 means or other become combined into groups and are 

 capable of giving rise to the various parts and organs in 

 question. Then, and then only, would the theory serve as 

 an incentive to further investigations into the phenomena of 

 heredity of all kinds, and experiments might be made which 

 would support or contradict it. 



There is no doubt a natural tendency to base experiments 

 upon certain preconceived ideas ; but it is one thing to be 

 guided solely by such phenomena as may at the moment 

 appear of especial importance, and another to base opera- 

 tions upon the completed outline of a theory founded upon 



